The present invention is directed to improved superabsorbent polymers demonstrating high fracture resistance. More particularly, the present invention is directed to improved superabsorbent polymers containing a dispersed phase comprising an elastomeric material. The resultant improved superabsorbent polymer is particularly useful in disposable personal care articles such as diapers, adult incontinence devices, and feminine napkins.
Conventional absorbent articles such as baby diapers, adult incontinence devices, and feminine napkins are typically made with a cellulose fiber fluff-based absorbent core sandwiched between a liquid pervious top sheet whose function is to allow the unobstructed passage of fluid to the absorbent core, and a liquid impervious backing sheet, usually of plastic material, whose function is to contain the absorbed fluid and prevent it from passing through the absorbent core and soiling the undergarments of the wearer of the absorbent article.
The absorbent core of these absorbent articles is typically constructed of defiberized wood pulp combined with superabsorbent polymer granules. The absorbent core is typically formed on a carrier tissue in a pad forming unit of a converting machine. With regard to conventionally produced absorbent structures, reference is made to U.S. Pat. Nos. 5,009,650, 5,378,528, 5,128,082, 5,607,414, 5,147,343, 5,149,335, 5,522,810, 5,041,104, 5,176,668, 5,389,181, and 4,596,567, the disclosures of which are hereby incorporated herein by reference, as are the disclosures of all other patents, patent applications or references cited herein.
It is known from U.S. Pat. Nos. 3,669,103 and 3,670,731 that carboxylic polyelectrolytes may be crosslinked to create hydrogel-forming materials, now commonly referred to as superabsorbents, supersorbers or superabsorbent polymers (generally referred to herein as xe2x80x9cSAPsxe2x80x9d), and to employ such materials to enhance the absorbency of disposable absorbent articles. It is also known from U.S. Patent Nos. 3,980,663 and 4,076,673 that SAP may be formed by adding crosslinkers to solutions of carboxylated polyelectrolytes and then drying and curing the polymer. Unfortunately, the prior art approaches to making SAPs yield brittle, glassy, abrasive particulates. As a result, absorbent products generally incorporate SAPs in the form of discrete particles which may take the form of granules, flakes, powder, chunks, nuggets, pellets, needles, fibers, rods and the like. During further handling or processing, these brittle SAP materials tend to break into smaller particles, even dust particles that are small enough to become airborne. The fracture of SAP particles into smaller particles or dust creates an industrial hygiene problem. The airborne dust contaminates the air in the manufacturing and converting plants. In addition, small particles can foul the manufacturing and converting equipment. Sometimes fouling problems necessitate the incorporation of design features (such as carrier tissue) in the absorbent products simply to minimize the fouling of the equipment by small SAP particles. The brittleness of SAPs has traditionally poses a dusting problem not only while it is being processed into an absorbent article, but also in that the particle size distribution shifts toward smaller particles after the product leaves the manufacturer while it is being processed into an absorbent article.
Work place dust is handled by engineering controls such as air filtration and dust collection systems, but it would be better to eliminate or at least substantially reduce the formation of dust so that the engineering controls in the work place would be a secondary line of defense and not the primary dust-control means.
Accordingly, it would be desirable to provide a plastic SAP particle, which would be more resistant to fracture and disintegration during processing. Humectants, such as glycerol, have been suggested as plasticizers for SAPs. However, because they function well only in the presence of water, their use is impractical as the SAP becomes tacky in the presence of small amounts of water, and is designed to swell and gel in the presence of large amounts of water. Furthermore, water is not a practical plasticizer for SAPs in a commercial setting because the moisture level in the polymer and, consequently, its ductility or brittleness would fluctuate with changes in the relative humidity.
Another proposed dust control technique entails coating the brittle SAP particles with a non-penetrating hydrophilic liquid which would trap the microscopic dust particles as they form and prevent them from becoming airborne during subsequent handling. This approach is described in WO 94/22940, in which various polyethylene oxide adducts used in surface treatments of SAP particles are taught as de-dusting agents. A different approach is disclosed in EP 0 690 077 Al, wherein the fracture mechanics of the SAP particles are modified by using polyethylene oxide functional co-monomers in the conventional acrylic acid polymerization or through a process of solvent exchange, to distribute polyglycols throughout superabsorbent granules. The polyglycol materials are effective in minimizing the generation of polymer fines and aerosol dusts when the dry particles are subjected to high impact and shearing conditions. However, such polyglycol co-monomers are costly specialty chemicals and using them at levels high enough to effectively modify SAP properties would make the resultant SAP economically unattractive. Even the high molecular weight polyethylene oxide polymers which could be solvent-exchanged into the granules cost several times that of the SAP itself.
Accordingly, there is a need in the art for novel approaches for solving the brittleness problems of SAPs. Applicant has now surprisingly discovered that an aqueous dispersion of a rubbery material may be used to successfully overcome the brittleness problem.
The present invention is directed to a hydrogel-forming polymeric material including a first compound which is a superabsorbent polymer and a second compound including an elastomeric material (i.e. , soft, rubbery latex) effective to increase the fracture resistance of the superabsorbent. The second compound is present in the first as a dispersed phase. The first compound includes a water insoluble but water swellable polymer or a carboxylic-functional polyelectrolyte in combination with a crosslinker reactive with carboxyl or carboxylate groups. The second compound includes an effective amount of an aqueous dispersion of a rubbery material.
All patents, patent applications and references cited herein are incorporated hereby by reference. In case of inconsistencies, the present disclosure governs.
The present invention provides a cost-effective, improved superabsorbent material having modified fracture mechanics to greatly minimize the formation of dust during the handling of the material. This is accomplished by thoroughly blending the superabsorbent polymer (SAP), at the time of its manufacture, with an elastomeric material which forms a dispersed elastomeric phase thereby lowering the ductile-brittle transition temperature of the SAP to below room temperature so that frequency of spontaneous particle fracture during shipping and handling is greatly diminished. The elastomeric material used in the present invention is a soft latex that has a rubbery appearance upon drying. In the context of this invention, a soft latex is one which is film-forming at room temperature unlike a hard latex which dries to a powder and must be heated above its softening point to become film-forming.
The hydrogel-forming material of the present invention is a blend of at least two immiscible polymersxe2x80x94(1) the superabsorbent polymer (SAP), and (2) the polymer present in an aqueous dispersion of an elastomer, which is combined with the SAP during manufacture. As used herein, the xe2x80x9chydrogel-forming polymeric materialxe2x80x9d refers to a highly absorbent material having a property of forming a gel upon liquid absorption. A xe2x80x9csuperabsorbent polymerxe2x80x9d is a water soluble polymer which has been crosslinked (for example upon drying and curing with a crosslinkers reactive with a functional group on the polymer or by including a poly-functional comonomer in the polymerization recipe) to render it water insoluble but water swellable.
The preferred SAP of the invention contains a carboxylic-functional polyelectrolyte in combination with a crosslinkers reactive with carboxyl or carboxylate groups. In addition to the SAPs described in the Example, it is contemplated that any polymer which would be water soluble if not crosslinked may be employed. Suitable examples include polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, carboxymethyl cellulose, polyvinylmorpholinone, polymers and copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides, polyvinyl pyridine and the like.
Crosslinking agents that may be used for preparing the hydrogel-forming polymeric material of the invention are well known in the art.
Illustrative examples of the polyfunctional crosslinking agents useful in this invention to convert the above water soluble polymers into polyelectrolytes into water-swellable polymers are set forth in U.S. Pat. Nos. 2,929,154; 3,224,986; 3,332,909; and 4,076,673. These polyfunctional crosslinking agents are generally known as polyamide-polyamine epichlorohydrin adducts. Similar crosslinking agents such as commercially available Kymene 557 and Polycup 172 (obtained from Hercules Incorporated, Wilmington, Delaware). The structure of these adducts is well known and is described in M. E. Coor et al., Journal of Applied Polymer Science, Vol . 17, pages 721-735 (1973).
Illustrative examples of the difunctional agents useful in this invention are polyhaloalkanols such as 1,3-dichloroisopropanol; 1,3-dibromoisopropanol; sulfonium zwitterions such as the tetrahydrothiophene adduct of novolac resins; haloepoxyalkanes such as epichlorohydrin, epibromohydrin, 2-methyl epichlorohydrin and epiiodohydrin; polyglycidyl ethers such as 1,4-butanediol diglycidyl ether, glycerine-1,3-diglycidyl ether, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, polypropylene glycol diglycidyl ethers having an epoxy equivalent weight range from about 175 to about 380, bisphenol A-epichlorohydrin epoxy resins having an epoxy equivalent weight range from about 182 to about 975 and mixtures of the foregoing.
Also useful as crosslinking agents are monomeric amine-epihalohydrin adducts. Sulfonium zwitterions described in U.S. Pat. Nos. 3,660,43 1; 3,749,737; and 3,749,738, may also be used.
The crosslinking agents may be used in an amount from about 0.05 to about 5.0 percent based on the weight of the polyelectrolyte used. This is generally sufficient to cause the polyelectrolyte to become lightly crosslinked. However, this range may vary for each polyelectrolyte in order to adjust the absorbency of the final crosslinked material and can be determined using routine experimentation. Hydrogel-forming polymer may be prepared directly from crosslinking monomers, such as for example, N,N-methylenebisacrylamide, ethylene glycol diacrylate, triallyl amine, and trimethylolpropane triacrylate. The crosslinking comonomer suitable for use in the present invention has at least two sites of unsaturation capable of copolymerizing with the backbone comonomers during radical-induced polymerization. An especially preferred hydrogel-forming polymer for this invention is crosslinked partially neutralized poly(acrylic acid).
The second component of the hydrogel-forming material of the invention is an elastomeric material which acts to increase the fracture resistance of the SAP. Elastomeric material is added to SAP in an effective amount as an aqueous dispersion. As used herein, an xe2x80x9ceffective amountxe2x80x9d is that amount of elastomer that is effective to increase the fracture resistance of SAP. In one embodiment of the invention, the effective amount is from about 1.0% to about 50% , preferably from about 2.0% to about 25%, and most preferably from about 5% to about 20%.
Representative examples of suitable elastomeric materials are the natural and synthetic latexes which are commonly used as binders and elastomeric adhesives in the fabrication of airlaid absorbent products. In addition to the latexes described in the Examples, its is contemplated that any natural or synthetic elastomer capable of forming a latex dispersion would be suitable for use in the present invention. Thus, natural rubber, polybutadiene rubber, styrene-butadiene rubber, acrylonitrile butadiene rubber, poly-2-chlorobutadiene rubber, polyisoprene rubber, isopreneisobutylene copolymers, ethylene-propylene rubber, ethylene-vinylacetate copolymers, chlorinated polyethylene, chlorosulfonated. polyethylene, acrylic rubber, ethylene-acrylate copolymers, epichlorohydrin rubber, polypropylene oxide rubber and polyurethanes, may be used.
One skilled in the art of polymer processing may prepare the dispersed elastomeric phase of the present invention by emulsifying a solution of an appropriate elastomer in an organic solvent in the aqueous charge to a polymerization reactor followed by stripping the organic solvent prior to charging the monomers. It is additionally understood that any xe2x80x9csoftxe2x80x9d or elastomeric composition would fall under the scope of this invention, even those which depend on the use of tackifier resins to attain their softness.
In one embodiment of the invention, the hydrogel-forming polymeric material exhibits a gel capacity of at least about 20 grams of 0.9% saline per gram of material and a Friability Index of at least about 25 percent greater than the Friability Index exhibited by an otherwise substantially identical hydrogel-forming polymeric material that is prepared without an elastomer.
The improved hydrogel-forming polymeric material of the present invention may be prepared in at least two ways. In one embodiment, the elastomeric material may be blended with a water soluble pre-polymer, and the pre-polymer is then crosslinked to form the SAP. In another embodiment, the elastomeric material may be blended into a monomer prior to conventional superabsorbent gel polymerization. The elastomer addition must be done before the superabsorbent is crosslinked for example by mixing the elastomeric with the monomer/crosslinking comonomer solution or by mixing the elastomeric with the water soluble polymer/crosslinkers solution.
In one embodiment, a carboxylic-functional polyelectrolyte solution may be prepared by: (i) blending a crosslinkers reactive with carboxyl or carboxylate groups into the polyelectrolyte solution along with an aqueous colloidal dispersion of a rubbery polymeric material; and (ii) drying and sizing the hydrogel-forming polymeric material wherein the particle size distribution is between 75 and 1000 microns and is useful as the superabsorbent additive to absorbent hygienic products.
In another embodiment, a process for preparing a hydrogel-forming material comprises the steps of:
(i) preparing an aqueous solution of carboxylic-functional ethylenically unsaturated monomers along with a suitable polyfunctional crosslinking agent either reactive with the carboxyl or carboxylate group or reactive in radical-induced copolymerization with the aforementioned monomer solution;
(ii) blending into the monomer solution of step (i) an amount of an aqueous colloidal dispersion of a elastomeric material in the amount effective to increase the fracture resistance of the water swellable hydrogel-forming polymeric material;
(iii) initiating the polymerization of the monomer/elastomer blend of step (ii) by known radical-generating techniques; and
(iv) drying and sizing the hydrogel-forming polymeric material so that the particle size distribution is substantially within the range of between 75 and 1000 microns.
The invention further relates to a disposable absorbent product prepared with a fracture resistant hydrogel-forming material of the invention. In one embodiment, the disposable absorbent product of the invention comprises a liquid-permeable topsheet, a backsheet attached to the topsheet, and an absorbent structure positioned between the topsheet and backsheet, wherein the absorbent structure comprises a hydrogel-forming polymeric material of the invention. Absorbent articles of the invention may be prepared using techniques well known in the art.